The invention relates generally to marine seismic prospecting and more particularly to apparatus and methods for reducing the effects of undesired seismic reflections in sensors towed behind a survey vessel or in sensors laid on the sea bottom.
In towed marine seismic exploration, a hydrophone array is towed behind a marine vessel near the sea surface. The hydrophones reside in multiple sensor cables commonly referred to as streamers. A seismic source, also towed near the sea surface, periodically emits acoustic energy. This acoustic energy travels downward through the sea, reflects off underlying structures, and returns upward through the sea to the hydrophone array. The hydrophone array records the upward traveling seismic acoustic wave from the seabed. The hydrophone recordings are later processed into seismic images of the underlying structures.
Acoustic impedance is the product of the density ρ and the speed of sound c in a medium, ρc. Reflections occur any time a change in acoustic impedance is encountered by the sound waves. The greater the change in acoustic impedance, the more the energy is reflected. Since the acoustic impedance of air and water differ greatly, the sea surface is a nearly perfect reflector of sound energy. After returning from the sea bottom or the target of interest, the energy is again reflected by the sea surface back toward the streamer. Because a hydrophone has an omni-directional response, the hydrophone array records a ghost response, which is the seismic acoustic wave reflected from the sea surface and arriving delayed in time and reversed in polarity from the direct reflection. The ghost is a downward traveling seismic acoustic wave that, when added to the desired wave, detracts from the recorded seismic image.
The ghost produces a notch in the frequency spectrum of a hydrophone response at fnotch=c/2d, where c is the speed of sound and d is the streamer depth. Seismic streamers have been conventionally towed at a depth of 10 meters or less. At a depth of 10 m, the notch frequency fnotch is 75 Hz. A frequency response extending beyond 100 Hz is required for high seismic image resolution. Streamers are therefore sometimes towed at shallower depths to improve the resolution of a seismic image.
The ghost-causing reflection can also continue to the sea bottom or other strong reflector and be reflected back up to again interfere with the desired reflections and degrade the image. These reflections are commonly referred to as multiples.
Towing at shallow depths is problematic because noise from the sea surface interferes with the desired seismic signals. Furthermore, circular water currents near the sea surface can cause flow noise at the streamer skin. These effects are worsened as weather deteriorates, sometimes causing the crew to discontinue operations until the weather improves. The deeper the tow, the less sea-surface noise and weather are factors. If the ghost-notch effects can be eliminated, it is desirable to tow at greater depths.
Ocean-bottom, or seabed, systems, in which the seismic cable or sensors are placed on the seabed, reject ghosts by a technique commonly known as p-z summation. In an acoustic wave, the pressure p is a scalar and the particle velocity u is a vector. A hydrophone records the seismic acoustic wave pressure p, with a positive omni-directional response. A vertically oriented geophone or accelerometer records the vertical component of the seismic acoustic wave particle velocity uz, with a positive response to upgoing signals and a negative response to downgoing signals. In p-z summation, the velocity signal is scaled by the acoustic impedance ρc of seawater and added to the pressure signal. If an accelerometer is used, its output can be integrated to obtain the velocity signal, or the hydrophone signal can be differentiated so that it can better spectrally match the accelerometer signal. This produces a compound sensor that has full response to the upward traveling wave and zero response to the downward traveling wave to reject the ghost and multiples. One such method of signal conditioning and combination of signals to get a single deghosted trace is described in U.S. Pat. No. 6,539,308 by Monk et al. This and similar techniques work well when the particle-velocity sensor or accelerometer is not affected by unwanted motions due to factors not caused by the desired signal. Such unwanted accelerations are common in a seabed system deployed in a surf zone or area when there are strong bottom currents.
Recently there has been interest in using the combination of hydrophones and particle-motion measurement to reduce these effects in a seismic streamer. Operating a particle-motion sensor in a seismic streamer presents a problem because the streamer experiences accelerations due to towing or sea-surface effects that are large compared to accelerations caused by the desired reflections. Moreover, these unwanted accelerations are in the same spectral band as the desired reflection response.
Seismic streamers and seabed seismic cables experience all roll angles from 0° to 360° and moderate pitch angles. To implement a vertically oriented geophone, ocean-bottom systems have used: (a) a gimbaled moving-coil geophone; (b) a 3-component, omni-tilt moving-coil geophone with attitude sensing and computation external to the sensor to resolve the measurement relative to gravity; and (c) a 3-component, micro-electro-mechanical system (MEMS) accelerometer with internal attitude sensing and computation external to the sensor to resolve the measurement relative to gravity.
U.S. Pat. No. 7,167,413 to Rouquette uses an accelerometer in a seismic streamer to reject the ghost-notch effect. Rouquette uses a mass-spring system to reduce the effect of cable dynamics on the accelerometer and a load-cell system to measure and reject the cable-motion-induced noise on the accelerometer. The Rouquette system relies on well-known complex mechanical relationships that do not remain constant with manufacturing tolerances, aging, and environmental conditions. Rouquette uses a signal-processing adaptive algorithm to derive the relationship of the load-cell-sensor-and-mass-spring system to the acceleration acting on the accelerometer in situ. Rouquette describes a complex mechanical and electronic system.
U.S. Pat. No. 7,239,577 to Tenghamn et al. describes an apparatus and method for rejecting the ghost notch using an acoustic-wave particle-velocity sensor. Tenghamn et al. teaches the use of a fluid-damped, gimbaled geophone. It is known in the art that the fluid encapsulating the geophone is chosen to provide damping of the sensor swinging on its gimbals. While not described in Tenghamn et al., it is known in the art that a mass-spring vibration-isolation system can reduce the effect of cable mechanical motion on the geophone response. Motion of the geophone caused by cable mechanical motion is indistinguishable from acoustic-wave particle motion in the geophone response. The desired seismic-wave particle motion is obscured by cable mechanical motion in Tenghamn et al.
U.S. Pat. No. 7,359,283 to Vaage et al. describes a method of combining pressure sensors and particle-motion sensors to address the impact of mechanical motion on the particle-motion sensors. In this method, the response of the particle-motion sensor below a certain frequency fo is not used, but only estimated from the pressure-sensor response and the known pressure-sensor depth. The frequencies rejected are those for which mechanical motion of the streamer is expected. The estimated response has a poor signal-to-noise ratio at the lower frequencies of interest. This rejection below a certain frequency is not optimal as it also rejects valid signals in an important low-frequency band where deep-target data is likely to exist.
While the patents mentioned all describe methods to reject the ghost notch in a seismic streamer, all fall short of adequately accounting for the effects of streamer tow and current-induced motion on particle-motion sensors. All also fall short of producing high-fidelity, sensed acoustic-wave components with good signal-to-noise ratio down to the lowest frequencies of interest.